A black hole is a volume from which photons, or any matter, can not escape. More formally, the coordinate speed of light at the event horizon - the boundary of a black hole - is zero, as measured by a sufficiently separated observer.

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Schwarzschild Solution

I'm able to derive the Schwarzschild solution under the assumptions that the metric is (1) static (2) spherically symmetric and that the space is the vacuum. However, I have read that the ...
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Why we don't see time dilation in stars orbiting black hole?

We have images of stars orbiting black holes or black holes destroying near stars, but why do we see the stars moving normally? I mean, if time dilation does exist, shouldn't we see that stars slow ...
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How close would you have to be to the merger of two black holes, for the effects of gravitational waves to be detected without instruments?

Assume two black holes in the most common size range, spiraling into each other until they merge. The event releases significant amounts of energy via gravitational waves, which warp the space-time. ...
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Thermal radiation in the Unruh Effect

The following formula has been given in 't Hooft's black holes notes ($|\Omega \rangle$ is the vacuum state of Minkowski space, O is a operator): $$\langle \Omega| O|\Omega \rangle = \sum_{n \ge 0} ...
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Why do physicists trust black hole physics?

Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following: Black holes are predicted by General Relativity, a classical theory ...
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Information Retrieval

This question is motivated by the issue of information retrieval from black holes, but it is essentially a question about quantum information. It is widely believed (in certain circles) that the ...
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Do the stellar remnant in a black hole have spatial extent? [duplicate]

I am not talking about event horizons. I am talking about the actual remnant of stellar collapse. Is it just a point, hence the problem with the singularity? Or does it have a finite volume that we ...
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Definition of event horizon - Gravity around blackholes [closed]

Put simpler: A black hole is surrounded by a 'sphere' where, to an outside observer. the speed of light is (near) zero. What is the radius of that 'sphere' (for a non-rotating black hole) and how ...
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Is the event horizon of black holes visibly sharp, or blurry?

As you come in closer to a black hole, how do you see the event horizon? Is it always like a clear-cut surface? Or it only looks clear-cut from a distance, but as you come closer to the black hole, ...
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Theoretical relativity of black hole and antimatter [closed]

(Theoretically), can black holes be considered antimatter that just cancels its equivalent mass and goes back to neutral stage? Note: Be kind I am not a physicist .
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How can an infinitesimally small object rotate?

How is it possible for an object like a black hole or electron to spin since they are made up of no other components? If my understanding is correct, an electron is an elementary particle which means ...
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How can we detect a black hole accurately?

I know there are many ways in detecting a black hole, but there are odds against them too. So I wanted to know a accurate method to detect a black hole. Here are the known methods to me and the odds ...
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Spinning micro blackholes power conversion

In the context of energy extraction of spinning black holes, there are two known mechanisms: the Penrose process and the Blandford-Znajek process. The former relies on fragmentation of accreting flow, ...
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Black hole entropy

Bekenstein and Hawking derived the expression for black hole entropy as, $$ S_{BH}={c^3 A\over 4 G \hbar}. $$ We know from the hindsight that entropy has statistical interpretation. It is a measure ...
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Decay of matter

I was watching Stephen Hawking's documentary and in there he explained how he realized why black holes eventually disintegrate: There are ripples in space, an antiparticle and a particle get ...
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Where and how is the entropy of a black hole stored?

Where and how is the entropy of a black hole stored? Is it around the horizon? Most of the entanglement entropy across the event horizon lies within Planck distances of it and are short lived. Is ...
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Can we see the photon sphere from outside? [duplicate]

This question came to my mind when i saw the movie interstellar. In the movie there is a scene with a black hole and a sphere of light around it, what i assume to be the photon sphere. You can see it ...
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Do all stars Turn into Black Holes? [duplicate]

If all stars turn into black Holes one day, in far future, what will happen to Earth if Sun becomes a Black Hole.
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What constitutes a blackhole firewall?

I understand that, to break the entanglement of two particles of Hawking radiation and therefore preserve monogamy of entanglement, there should be a firewall around the event horizon. This firewall ...
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Possible paradox with black holes and time dilation [duplicate]

I am by no means an expert in the realm of physics. I do from time to time, try to understand the concepts of modern physics and their applications. I came across this video that I am currently ...
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How does a Black hole attract light? [duplicate]

Please no hate for lack of knowledge: I am somewhat fascinated with the subject of black holes. However, I do not understand a concept which is constantly attributed with black holes: that a black ...
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How does one correctly interpret the behavior of the heat capacity of a charged black hole?

Consider the Reissner-Nordström metric $$ds^2=-f(r)dt^2+f^{-1}(r)dr^2-r^2d\Omega^2\hspace{2cm} f(r)=1-\frac{2M}{r}+\frac{M^2q^2}{r^2}$$ where I defined the charge-to-mass ratio $q:=Q/M$, which ...
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How long can a particle survive inside of the horizon of a black hole?

By examining the causal structure of a Schwarzschild black hole, one can see that a particle in region II is unable to escape to $r=\infty$. Such diagrams do not show, however, how long a particle ...
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How does Hawking radiation grow as a black hole evaporates?

The temperature of Hawking radiation is inversely proportional to the mass of a black hole, $T_{\rm H}\propto M_{\rm BH}^{-1}$, and so as the black hole shrinks the temperature of the radiation should ...
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Generalized Bekenstein-Hawking temperature for Kerr-Newmann-dS black holes

What is the formula for the Bekenstein-Hawking temperature in a Schwarzschild-Kerr-Newmann-de Sitter spacetime, i.e., the temperature for a black hole with Mass (M), angular momentum (J), electric ...
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Hawking Temperature of the BTZ Black Hole

The metric of the BTZ Black Hole is given by $$ ds^2 = - N^2 dt^2 + N^{-2} dr^2 +r^2(d\phi + N^\phi dt)^2 $$ with $$ N^2 = -M+ \frac{r^2}{l^2} + \frac{J^2}{4 r^2}, \ \ \ \ \ \ N^\phi = ...
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Is the Hawking radiation of a charged black hole thermal?

Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation). Question: is it possible to throw a charge $Q$ at a faster rate than it will be re-radiated? Will ...
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How can a black hole produce sound?

I was reading this article from NASA -- it's NASA -- and literally found myself perplexed. The article describes the discovery that black holes emit a "note" that has physical ramifications on the ...
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global transformations in 3d gravity

I am currently working on proper and improper gauge transformations in 3d gravity and btz black holes. (because I have seen it defined with many different ways I will just say that with "proper"and ...
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Black Hole Growth

When a black hole increases in size by engulfing matter, does this matter fall into the event horizon or does it just rotate around the accretion disk? EDIT: Clarified the beginning of the question. ...
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Do black holes violate the Uncertainty Principle?

If black holes have mass but no size, does that imply zero uncertainty in position? If so, what does that imply for uncertainty in momentum?
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Connection between Black Hole Firewalls and Hawking Radiation

In a previous post, I asked what constitutes a black hole firewall--namely, what is a possible mathematical description of the firewall. Now I have a new question, which is based around this guest ...
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Problem with black holes?

I understand that black holes are formed when supermassive stars (>10 times the size of our Sun) die. When they die, their cores continue to shrink. I have read that eventually the core reaches a ...
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Size of black hole so large that I could pass event horizon without dying from tidal forces?

Were I to fall towards a typical black hole, the tidal forces would rip me apart well before I got to the event horizon. However, if a black hole were big enough, I could enter the event horizon ...
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Why don't electrons collaspe into black holes? [duplicate]

An electron has a mass of $9.10938291(40) \times 10^{−31} kg$. It also has a volume of $0 m^3$. This would imply it has infinite density. Shouldn't that make it collapse into a black hole? Why doesn't ...
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Can we observe a star collapse into a black hole? [duplicate]

Let's say that I happen by a star that has just burned out and is collasping into a black hole. I seat at infinity to watch it as a Schwarzschild observer. Will I ever see the black hole collapse into ...
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Free-fall path into a black hole in Kruskal Coordinates

If an object at t=0 begins to free-fall into a black hole from X in Kruskal coordinates (https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coordinates), what does its path on the Kruskal-Szekeres ...
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Black Hole surface area at Schwarzschild radius is half?

I have been interested in black holes for some time, and am still trying to wrap my head around some of their more obscure properties. Now I know that the Schwarzschild radius is $r= ...
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Thought Experiment - Poking a stick across a Black Hole's Event Horizon

The classical explanation of a black hole says that if you get to close, you reach a point - the event horizon radius - from which you cannot escape even travelling at the speed of light. Then they ...
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Any interesting implications of a holographic black hole in a holographic universe?

Bear with me; I don't have a background in physics, nor am I sure I even understand the things I'm talking about. I recently read that math/physics for a holographic black hole could be applied to ...
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Would infinite material cause a black hole?

If you have an infinite amount of any material(That doesn't have a critical mass to have nuclear reactions), would this matter form massive black holes that condense into an infinite black hole? Two ...
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Rigorous Proof of General Relativity's Non-renormalizability?

The answer to this question and the comments on it implies that general relativity has not been rigorously shown to be non-renormalizable for all loop diagrams -- only shown for two loops. However, ...
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Help me calculate the Euclidean action of a gravitating system!

I recently read Gibbons and Hawking's paper Action integrals and partition functions in quantum gravity, Phys. Rev. D 15 (1977) 2752. I am interested in repeating their calculations. It is fairly ...
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If a super massive black hole captures a very small black hole, would the smaller black hole be immune to spaghettification?

I think to answer this question we would have to fully understand the nature of black holes. It seems to me that the smaller black hole could have a lower density than the larger one. In this case the ...
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How to get Planck length

I know that what Planck length equals to. The first question is, how do you get the formula $$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length? The second question is, will any ...
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The uncertainty principle and black holes

What are the consequences of applying the uncertainty principle to black holes? Does the uncertainty principle need to be modified in the context of a black hole and if so what are the implications ...
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Does the uncertainty principle affect event horizons? [duplicate]

I was thinking about black holes. For a simple black hole the event horizon is given by a distance of 2 times mass (energy) of the black hole. (2m). But according to quantum mechanics, if you try to ...
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How can a singularity have infinite density? [duplicate]

I've heard many times that black holes have infinite density at their center, but I've also heard astronomers quote mass estimates for black holes. How could the singularity have infinite density if ...
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Wave equation on Schwarzschild background

I am trying to follow the solution of the wave equation for a scalar field on Schwarzschild background from http://batteringram.org/science/gr/scalar_wave.pdf. I have a problem on page 2 where they ...
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Which way does a black hole spin?

As far as I understand, and from what I have been shown in renderings of black holes, they spin (like water going down a drain). My question is, firstly, does the matter being pulled into a black ...